Resistances & Vulnerabilities - What can be learned?

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The scenario: the players encounter a monsters who has resistance and/or vulnerability to something or other. No players gets a monster knowledge high enough to know the resistance or vulnerability of the monster.

My question: to what extent, if any, can the players learn the monster's resistances and/or vulnerabilities by using powers with the corresponding keywords against it? Can they learn the exact number? Only the fact that they have a resistance/vulnerability to something specific? Or is it just a number that the DM silently subtracts/adds?

I've been unable to find something in the books, have I just missed it or is it something that needs to be houseruled?

Thanks for reading!
As far as I know the rules are silent on this issue, so use your best judgment.

I tell my players that their attack seems to be slightly less or more effective than they might have anticipated.
Me too that's what i do. 

And for vulnerability i portray it in a manner to indicate it seems to have suffered more. Its just a matter of game transparency for me.

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Bestairy - book from mordekains magnifcent emporium - allow a reroll of a monster knowledge check, after a short rest can be handy for the adventures in these sort of situations.
WotC's Transparency article advises "When the hell hound gets hit by scorching burst, tell the players that it has fire resistance" or at least 'describe the apparent results of the characters’ actions, allowing them to draw their own conclusions. “Even though the hell hound makes no effort to dodge your scorching burst, the flames of your spell wash over it to no effect.”'

Great, thanks for the answers guys!
I find as a general rule, less meta information the players know = more immersed in the game they become.
I find as a general rule, less meta information the players know = more immersed in the game they become.

I agree that saing "your fire doesn't affect it very much" is often better than saying "it has resist 10". But if a players asks, I don't mind telling them the actual number.

It only fair, since the PC's would have access to many more visual/tactile cues than I could ever relay to the players.